Implantable medical devices that respond to a patient's metabolic needs are known that include two different kinds of sensors, i.e. one sensor for the measurement of a corporal parameter that is predominantly physiologic, and one sensor for the measurement of a corporal parameter that is predominantly physical.
One will hereinafter refer to the particular example of a minute-ventilation (also sometimes known as “minute volume”) (MV) sensor as the physiologic sensor, corresponding to the most usual case, but it should be understood that this example is in no way limiting, and that other types of sensors also may be used, so long as they provide a signal that is representative of the patient's metabolic needs (for example, a sensor that is measuring blood oxygen saturation) or his hemodynamic status (for example, an intracardiac bioimpedance sensor) based on a physiological parameter. Likewise, one will hereinafter refer to the particular example of an acceleration (G) sensor as the physical (or activity) sensor, corresponding to the most usual case, but here, too, it should be understood that some other type of sensor can be considered, notably to detect the patient's movements.
Generally, the physical (activity) sensor is characterized by a response time that is shorter than that of the physiological sensor, in order to allow a very fast detection of short-duration activity, before a significant change of the physiological parameter, which varies more slowly, is detected.
The European patent EP 0750920, and its counterpart U.S. Pat. No. 5,722,996, commonly assigned herewith to ELA Medical, are proposing such a device that is enslaved, i.e. responsive to, two such sensors, operating a selection of one or the other sensor so as to take into account only the sensor that gives the more relevant signal at any given moment. The European patent EP 0919255 and its counterpart U.S. Pat. No. 6,336,048, commonly assigned herewith to ELA Medical, are proposing an enslavement based upon the use of a combination of the signals provided by these two sensors.
Besides enslavement, the signals provided by the sensors may be used for the purpose of diagnosing heart failure, so as to properly adjust the operation of the implanted device. Particularly, the European patent EP 0966987 and its U.S. counterpart U.S. Pat. No. 6,246,910, commonly assigned herewith to ELA Medical, allow to follow up evolution of the patient's condition over time so as to give an adequate representation of the patient's actual metabolic requirements, i.e. taking into account the actual activity level of the patient. The device described in that document adjusts its operation in case of a detected worsening or improvement of the patient's status, for example by reprogramming some of its functions, in order to follow the patient's evolution and adapt to the patient's effective cardiac decompensation level.
The starting point of the present invention lies in the inventor's observation of patients who, though actually implanted with improved devices, are adapting their own daily activity to the changes in their clinical status, with an incidence on their activity level, and, in some cases, on their respiratory status. Indeed, as it has been discovered, the clinical modifications are likely to be asymptomatic, and it is usual that the patient unconsciously adapts his activity to his clinical status: the first crisis of heart failure appearing during activity, the patient is led to reduce his activity in order to avoid the occurrence of such crisis. Hence, the symptoms no longer occur, the patient having changed his behavior so as to prevent them, but the pathology nevertheless keeps on evolving. Thus, the patient will go and visit his physician again, no sooner than when his heart failure will disturb him even when he is at rest, and in a worst case, the patient will have to turn to the emergency room for treatment.
In summary, due to such auto-adaptation, the absence of symptoms perceived by the patient leads to a significant delay between the onset of clinical modifications and the actual diagnosis thereof, the latter likely being too late.